Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 240345

Area Forecast Discussion
National Weather Service Seattle WA
845 PM PDT Fri Mar 23 2018

.SYNOPSIS...A cold, showery upper low off the coast will weaken
on Saturday with the remnant trough axis moving across Western
Washington Saturday night. Showers will taper on Sunday. On
Monday and Tuesday, a stalled warm front in northwest flow aloft
will lead to light to moderate rain. Brief upper ridging next
Wednesday and Thursday will lead to drier weather.


.SHORT TERM...An upper low will be off the coast tonight. This
will maintain a chance of showers. There will probably be an
uptick in the showers from the south after midnight as a short
wave moves through. Snow levels will be low enough for a threat
of snow over the higher lowland hills and areas away from Puget
Sound, especially later tonight. Accumulations, if any, should be
spotty with the showery nature of the precipitation and mostly on
things like grassy surfaces rather than roads. The best chance of
accumulations is over the East Puget Sound Lowlands towards the
Cascades early Saturday morning.

The upper low will sag south of the area Saturday and Sunday. A
weak upper trough will linger over the area with slightly unstable
air so a threat of showers will remain but the trend should be for
less showers. Highs will remain several degrees below normal
through the weekend. A weak warm front in the northwest flow
aloft will bring an increasing chance of rain Sunday night and
Monday. Schneider

.LONG TERM...Previous discussion...Tuesday will be similar to
Monday, with Western Washington under the dirty, drippy periphery
of an offshore upper ridge in northwest flow aloft. On Wednesday
and Thursday, the ridge axis will drift closer to the forecast
area, and 500 mb heights will creep up close to 570 decameters.
Cannot get rain out of the forecast completely, but Wed/Thu looks
quite a bit drier than Mon/Tue.

Models are in pretty good agreement with the northwest flow aloft
sweeping a sharp cold front southeast down Vancouver Island next
Friday. In the pre-frontal environment, warm advection and onshore
flow will serve to increase precip chances next Friday. Haner


.AVIATION...An upper low well offshore will shift south tonight
and Saturday with southerly flow aloft over Western Washington
easing and becoming light westerly. The air mass is somewhat moist
and slightly unstable.

Isolated to scattered showers will continue tonight and Saturday.
Ceilings will be mostly VFR with MVFR possible at times in
heavier showers. An upper short wave will bring a period of more
numerous showers and lower ceilings later tonight and Saturday

KSEA...VFR ceilings are expected this evening - 6000-10000 feet.
Ceilings could briefly lower to MVFR in heavier showers however.
MVFR ceilings are more likely late tonight as a more organized
batch of showers moves up from the south. Rain could mix with snow
for a few hours early Saturday morning but accumulations are not
expected at the terminal. Southerly wind 5-10 knots. Schneider


.MARINE...Low pressure offshore will shift south tonight and
Saturday with easing winds. A frontal system will slowly shift
through the area later Sunday and Monday. Moderate onshore flow
will develop behind the front Tuesday and Wednesday. West swell
around 10 feet over the Coastal Waters should fall below 10 feet
this evening. Schneider


.HYDROLOGY...River flooding is not expected for the next 7 days.


PZ...Small Craft Advisory for rough bar until 11 PM PDT this evening
     for Grays Harbor Bar.


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